The present invention relates generally to tube leakage testing and more particularly relates to tube leakage testing in moisture-separator reheat superheaters.
In nuclear power plants having light water reactors, the steam after expansion in the high pressure turbine may contain about 10 to 15% moisture. Accordingly, so as to avoid blade erosion and efficiency losses, the steam must be dried following the expansion in the high pressure turbine and subsequently superheated with live steam. These two processes typically take place within a moisture-separator reheat superheater.
From the functional aspects, moisture-separator reheat superheaters are vital and (owing to their size) quite expensive components of the turbine plant. For this reason, test capabilities are usually provided so that damage, i.e., lack of tightness in the superheater tube bundle, can be recognized during operation. This damage is preferably discovered while in the initial stage in order to thereby avoid secondary damages.
A conventional test of today is the shutdown test method in which the moisture-separator reheat superheater in a shutdown plant is investigated for possible damage by hydraulic pressure testing and visual examination. In the event that leaky tubes are present, these tubes are mended, i.e., the tube is shut down while its ends are closed at the tube sheets.
In order to determine the presence of leak points without placing the entire plant or the suspected plant component out of operation, consideration has been given to a continuous testing by accounting for a material balance or by a comparison of superheater temperatures. Unfortunately, these methods have the disadvantage, however, that their results are typically load-dependent and are relatively inaccurate in spite of a very considerable expenditure. In addition, after a certain period of time and also after overhauls, deviations from the status of the component when new appear in the reading. Furthermore, it would be advantageous to be able to quickly ascertain damages which appear suddenly such as a complete tube fracture. Even here, however, it is necessary for a number of "complete tube fractures" to occur before a reliable indication is given.
A primary object of the present invention is to provide a method and device with which essentially greater indicator accuracies in a leak cross section ratio can be achieved than in accordance with the known methods and devices.